1. Field of the Invention
The invention relates to a method for producing a single crystal by means of the floating zone method, and to an apparatus suitable for application of the method.
2. Background Art
On an industrial scale, the floating zone method is used, in particular, for producing single crystals composed of silicon. For this purpose, polycrystalline silicon is inductively melted and crystallized on a monocrystalline seed crystal. The polycrystalline silicon is usually provided in the form of a feed rod, which, starting from its lower end, is gradually melted by means of an induction heating coil, wherein a melt zone composed of molten silicon that forms serves for the growth of the single crystal. This method is referred to hereinafter as the FZ method.
One variant of the FZ method, which is referred to as the GFZ method hereinafter, uses polycrystalline granular silicon instead of a feed rod. While the FZ method makes use of one induction heating coil for melting the feed rod and for the controlled crystallization of the single crystal, the GFZ method makes use of two induction heating coils. The polycrystalline granules are melted with the aid of a first induction heating coil on a plate and subsequently flow through a hole in the center of the plate to the growing single crystal and form a melt zone. The crystallization of the single crystal is controlled with the aid of a second induction heating coil, which is arranged below the first induction heating coil. Further details concerning the GFZ method are described, for example, in US 2011/0095018 A1.
DE 30 07 377 A1 describes the FZ method and an apparatus suitable for carrying out the latter, wherein the description is devoted to the problem of preventing the occurrence of thermal stress. In order to solve the problem, it is proposed to reheat the single crystal by thermal radiation of a reflective protective sheath surrounding the single crystal. The publication by A. Muiznieks et al. (Journal of Crystal Growth 230(2001), 305-313) confirms the efficacy of a protective sheath—designated therein as a reflector—for reducing thermal stresses. Simulation calculations also show that the thermal stresses are highest in the center of the crystallization boundary and that their contribution increases with the diameter of the single crystal. Furthermore, it is shown that thermal stress, particularly in the center of the crystallization boundary, increases as the extent to which the crystallization boundary is bent toward the single crystal increases, and that this bending increases with the rate of crystallization of the single crystal.
Therefore, there is a need for measures which are directed against bending of the crystallization boundary, without having to restrict the rate of crystallization, and which are suitable for inhibiting thermal stress and the ensuing risk of the formation of dislocations without loss of productivity.
Since a reflector surrounding the single crystal impedes the heat transfer via the lateral surface of the single crystal, less heat has to be supplied to the melt zone via the induction heating coil in order to ensure a height of the melt zone that is required for crystal growth. This is a further advantage associated with the use of a reflector.
As the diameter of the single crystal increases and the rate of crystallization increases, the bending of the crystallization boundary increases. This aggravates the problems on account of thermal stresses. The use of a reflector then no longer suffices as a countermeasure.